In the mid-eighties, Davies was commissioned to write a concise history of Wales by Penguin Books to add to its Pelican series of the histories of nations. The decision by Penguin to commission the volume in Welsh was "unexpected and highly commendable," wrote Davies.[3]

"I seized the opportunity to write of Wales and the Welsh. When I had finished, I had a typescript which was almost three times larger than the original commission," wrote Davies.[3] The original voluminous typescript was first published in hardback under the Allen Lane imprint. Davies took a sabbatical from his post at the University College of Wales and wrote most of the chapters while touring Europe.[3] Davies dedicated Hanes Cymru to his wife, Janet Mackenzie Davies.[3]

Hanes Cymru was translated into English and published in 1993, as there was "a demand among English-speakers to read what was already available to Welsh-speakers," wrote Davies.[3] A revised edition was published (in both languages) in 2007

Davies lived in Grangetown, Cardiff. To mark his 75th birthday in 2013, the Welsh language television channel S4C broadcast a programme, Gwirionedd y Galon: Dr John Davies, about his life and his home.[4] He died at the age of 76 in 2015.[5]